Image sensors, as semiconductor devices converting an optical image into an electrical signal, may be largely classified into charge coupled device (CCD) image sensors and complementary metal oxide silicon (CMOS) image sensors. CMOS image sensors capture an image by sequentially detecting and switching electrical signals of unit pixels. A unit pixel may include at least a photodiode and a MOS transistor.
To attain bright and clear image quality in the CMOS image sensor, light should be maximally focused on the photodiode. A microlens, which may be made out of a photoresist, may be used for this. However, since the photoresist is not very hard and solid, defects may occur in the microlens involving silicon particles generated during wafer sawing. To guard against this problem, un-doped silicate glass (USG) is deposited on the microlens at low temperature. This may be called low temperature oxide (LTO).
Meanwhile, since an oxide material formed at low temperature is a sparse film having many pores therein, the oxide material may be cracked by thermal stress generated in a subsequent process. As an example, a diagram of a photo showing a case in which a pixel area is cracked after a deterioration reliability test of the image sensor is shown in FIG. 10. As a result, the reliability of the device is degraded.